FIELD OF THE INVENTION
The invention relates to a turbine casing having a multilayer casing wall with a pressure-tight, heat-insulating intermediate layer between an inner layer sealing off a pressure space and a force-transmitting outer layer. The invention also relates to a method of manufacturing such a casing. The term turbine casing refers in particular to the outer casing of a high-pressure steam turbine.
In a turbine working as a prime mover, potential energy of a flowing working medium such as, for example, gas or steam, is converted into mechanical work. To that end, the turbine includes a rotor wheel and a fixed guide wheel as essential elements. Thus, steam serving as a flow medium in a steam turbine is expanded until it condenses to perform work. In that case, the structure of the steam turbine is determined in particular by steam states, i.e. steam pressure and steam temperature.
Especially high steam states are aimed at due to the desire for an efficiency of a steam turbine which is as high as possible. In a high-pressure turbine, an increase in the live-steam pressure, e.g. to 300 bar, and in the live-steam temperature, e.g. to 600.degree. C., requires a material selection corresponding to the temperature effect and a turbine-casing wall thickness corresponding to the stress due to the internal pressure prevailing at high temperature. In that case, it should be taken into account that the admissible stresses markedly decrease with increasing component temperature. A correspondingly larger wall thickness would therefore be necessary to absorb the pressure forces with the turbine casing.
The casing parts that are required for high steam states and are made of temperature-resistant materials having a large wall thickness result in considerable material costs, in view of the high costs for such materials. However, ease of manufacture is also an aspect which proves to be an obstacle to an increase in the wall thickness, in particular the castability of the alloys at the requisite wall thicknesses. Further aspects to be taken into account are the operating behavior of the turbine with regard to the start-up and shutdown times influenced by the heating-up and cooling-down behavior of the casing parts and the handling due to the mass, which increases with the wall thickness. It should also be taken into account that, in the turbine casing, not only does the wall thickness increase with increasing pressure, but the strength of the conventional materials also decreases with increasing temperature.
In order to provide the heat insulation of an outer casing of a high-pressure turbine, it is known from German Published, Non-Prosecuted Patent Application DE 195 35 227 A1 to place an insulating layer or course on the inside of the outer casing, and to provide the inside of the insulating layer with a lining. In that casing wall having a multilayer construction, a castable ceramic or a castable lightweight refractory concrete is provided as an insulating layer. However, a disadvantage of that construction is that the hardened insulating layer tends to fracture as a result of operationally induced thermal stresses. That may lead in an undesirable manner to the formation of cracks in the adjacent layers, in particular in the outer layer.
Furthermore, it is known from Austrian Patent 381 367 B to provide a metallic insulating body, in particular in the form of metal fibers, in the steam space of a steam turbine. Since the insulating body provided in the steam space comes into direct contact with the steam serving as an insulating medium, on one hand the metal parts must be sufficiently large in order not to be entrained by the steam flow, which would lead to the destruction of the turbine. On the other hand, a sufficiently loose bulk fill of the metal parts is required so that the steam can flow at least more or less unimpeded through the insulating body. Sufficient heat insulation and pressure tightness is not achieved with such a metallic insulating body.